COLVILLE, Wash. – An 11-year-old boy was convicted Friday of conspiracy to commit first-degree murder in a fifth-grade plot that targeted a female classmate.

Stevens County Superior Court Judge Allen Nielsen said Friday that "simple anger" fueled the plot the boy hatched earlier this year with a 10-year-old classmate at Fort Colville Elementary school in northeast Washington.

School staff seized a handgun and knife the boys brought to school Feb. 7.

The judge, who called the trial "the most serious of my career," rejected defense efforts to portray the boy as unable to separate fact from fiction, The Spokesman-Review reported (http://is.gd/x8fwaO ).

The 11-year-old was led from court in tears. He's due back in court Nov. 8 for a sentencing hearing.

A defense lawyer told KHQ-TV an appeal is planned.

"There is no joy in a conviction," said Stevens County Prosecutor Tim Rasmussen, who added he was relieved by the verdict.

School counselor Debbie Rogers testified Friday about her interview with the 11-year-old the day the gun and knife were discovered. The boy said he was planning to stab the girl to death because she was "really annoying" and the second boy was to point the gun at anyone who tried to intervene, Rogers said.

Rogers said she saw no evidence that the older boy was experiencing delusions that day.

The younger boy pleaded guilty earlier to conspiracy to commit murder and related charges. He was sentenced to three to five years in a juvenile detention facility.

Authorities discovered the plan when a fourth-grader saw one of the boys playing with a knife aboard a school bus and told a school employee what he'd seen. A search of the 10-year-old's backpack found a knife, a .45-caliber semi-automatic pistol and a full ammunition magazine, court records showed.

The counselor's interview provided sufficient evidence to prove the two boys had developed a plan and were ready to carry it out when the weapons were found, the judge said.

Lawyers questioned a Boise-based forensic psychologist for more than two hours of Friday's court session. Craig Beaver had examined the 11-year-old multiple times for the defense.

The boy's behavior was "strongly suggestive of what we see with childhood bipolar disorder, which is relatively rare," Beaver said.

The judge found that defense lawyers successfully argued that the boy displayed some symptoms of bipolar disorder, but Nielsen said the child's behavior on Feb. 7 did not indicate any lack of awareness of the consequences of his actions.